Faraday shield suppressor for secondary emission current in crossed electric and magnetic field electronic tubes



Sept. 8, 1964 J. H. GONCZ 3,148,298

FARADAY SHIELD SUPPRESSOR FOR SECONDARY EMISSION CURRENT IN CROSSEDELECTRIC AND MAGNETIC FIELD ELECTRONIC TUBES Filed Jan. 9, 1962 2Sheets-Sheet l FIG.2

0A-- OUTPUT FIG.

JOHN H. GONCZ INVENTOR.

AGENT Sept. 8, 1964 J. H. GONCZ 3,148,298

FARADAY SHIELD SUPPRESSOR FOR SECONDARY EMISSION CURRENT IN CROSSEDELECTRIC AND MAGNETIC FIELD ELECTRONIC TUBES Filed Jan. 9, 1962 2Sheets-Sheet 2 PRIMARY 4 ELECTRON I SECONDARY 1 ELECTRON d l l I LCYCLOIDAL l PATH r CIRCULAR PATHS FIG. 3

PR|MARY SECONDARY ELECTRON ELECTRON PATH LL//////// N /7 CIRCULAR PATHSFIG. 4

JOHN HQGONCZ INVENTOR.

AGENT United States Patent FARADAY SHIELD SUPPRESSQR FDR SECGND- ARYEMESSIQN CURRENT EN CRQSSED ELEC- TRHC AND MAGNETIC FIELD ELECTRONICTUBES John H. Goncz, Waltham, Mass, wignor to Edgertou,

Germeshausen 8r Grier, Inn, Boston, Mass, 21 corporation ofMassachusetts Filed Ban. 9, 1962, Ser. No. 165,195 4 Claims. (til.313-165) This invention relates to a method and a device for controllingthe secondary electron emission in electron tubes where primaryelectrons are directed toward and impinge upon an electrode, and moreparticularly for suppressing secondary electron emission from anelectrode of tubes in which the electron flow is subjected to crossedelectric and magnetic fields.

Secondary electron emission from an electrode such as the anoderepresents a substantial energy loss in an electron tube and it isparticularly serious in tubes having crossed electric and magneticfields which cause the electrons to how in a cycloidal path. Priorsolutions to this problem have included (1) carbonizing the anode, (2)placing a suppressor grid of negative polarity with respect to the anodein the vicinity of the anode electrode, and (3) forming the anode of anirregular shape to trap secondary electrons that are produced. Each ofthese solutions possess certain disadvantages not found in the presentinvention.

In carbonizing the anode, or applying a plurality of black layersthereto, the electrons are trapped in a labyrinth of small particles.This method, however, is only successful in reducing secondary emissionto about 50% of the number of primary electrons striking the anode. Itis a serious disadvantage of this method that it is only 50% eifective.

By placing a suppressor grid having a negative potential with respect tothe anode adjacent to the anode, secondary emission is substantiallyreduced over carbonized anodes but undesirable currents are induced inthe anode by the motion of the electrons in the electric field betweenthe two electrodes. These currents cause an energy loss whichneutralizes the gain obtained by the reduction of secondary emission.Furthermore, a time smear, or loss of high frequency response is broughtabout by the motion of the electrons.

The third method which has been employed in the prior art, is shapingthe anode electrode to such a configuration that it has a series ofdepressions so disposed that the secondary electrons, emitted upon theimpingement of the primary electrons, also strike a portion of the anodeemitting tertiary electrons. Due to the reduced energy of the secondaryelectrons over that of the primary electrons, and the reduced energy ofthe tertiary electrons over the secondary, the secondary, tertiary andfurther orders of electrons are trapped and prevented from leaving theanode. This system has greatly reduced energy loss but to accomplishthis end, a complicated and expensive anode configuration is requiredwhich greatly increases the cost of producing such a device.Furthermore, such configurations still possess areas from whichsecondary emission takes place since the flow of primary electronscannot be so controlled that they strike the most advantageous points onthe anode surface.

It is an object of this invention to provide an electrode structurewhich will control the secondary electron emission from the electrode togreatly reduce energy losses therefrom without the above-mentioneddisadvantages.

Another object of this invention is to provide a simple inexpensiveanode structure that will greatly reduce energy losses due to secondaryemission from the anode.

A further object of this invention is to provide a new 3,148,298Patented Sept. 8, 1964 and novel method of reclucin energy losses causedby secondary electron emission from an electrode.

In summary, this invention consists of making the space immediately infront of an electrode free of electric fields while not affecting anymagnetic fields that may be present. The electric field-free spaceextends a distance normal to the electrode which is less than the radiusof the circular path described by the primary electrons in the existingmagnetic field but greater than the radius of the circular path of thesecondary electrons. One device for carrying out this method is amagnetic Faraday cage or shield spaced the above-defined distance fromthe electrode and energized to the same potential as the electrode.

Faraday cages have been used very successfully in electrostatic fieldsas efficient collectors of electrons. By this invention, it is used forthe first time in a magnetic field, and an efiicient, simple andinexpensive suppressor of secondary electron emission losses is producedthereby.

Other and further objects of this invention will be more particularlypointed out in the following description and in the appended claims.

The invention will now be described in connection with the accompanyingdrawing in which:

FIGURE 1 is a schematic view of an electron tube embodying the subjectinvention;

FIGURE 2 is an end view of the tube of FIGURE 1;

FIGURE 3 is an explanatory diagrammatic view of the anode electrode areaof an electron tube; and

FIGURE 4 is a view similar to that of FIGURE 3 but also including themagnetic Faraday cage.

The tube shown in FIGURE 1 is an amplifier tube having crossed electricand magnetic fields. This tube has within its envelope 1, a cathode 2, acontrol grid 3. a series of secondary electron emitting dynodes 4, 5 and6, an anode 7, a Faraday cage 8 and a high-voltage rail 9. The cathode 2is connected to the grounded side of a high voltage supply shown asbattery B. The input signal is fed to the control grid 3 which causesthe flow of electrons from the cathode 2 to dynode 4 which acts as aplate for the triode section of the tube. The electron flow is caused tofollow a cycloidal path due to the combined effect of crossed magneticand electric fields. The magnetic field in the path of the electron flowis produced by a pair of magnets N and S externally mounted adjacent tothe tube envelope 1 as shown in FIGURE 2. The electric field is producedby the high-voltage rail 9 connected to battery B These two fields, themagnetic field produced by the magnets N and S, and the electric fieldproduced by the high-voltage rail 9, are mutually perpendicular andcombine to cause the electrons to flow in a cycloidal path from thecathode 2 to the dynode 4. Each of the dynodes 4, 5 and 6 have asecondary-electron emitting surface such as, for example, magnesiumoxide, in order to multiply the electrons by emitting a plurality ofsecondary electrons for each primary electron striking the dynode. Theelectrons emitted by dynode 4 follow a cycloidal path to dynode 5 wherean increased number of electrons are emitted which flow to dynode 6where the multiplying effect again takes place causing the electrons toflow to the anode 7. The anode 7 is connected to the high-voltage sideof the battery B. Each of the dynodes 4, 5 and 6 and the anode 7 areheld at progressively higher potentials by the voltage divider R R R andR connected between the grounded cathode 2 and the anode 7.

Referring now to FIGURE 3, the anode 7 is a smooth planar electrode of aconducting metal such as, for example, copper or the like. It isimportant to reduce secondary emission from the anode 7 to as low alevel as possible to prevent energy loss and to realize in the output asmuch of the amplification produced within the tube as possible. This maybe accomplished by making the space dotted lines shown in FIGURE 3.

in front of the anode 7 free of electric fields and par-ticularly theelectric field produced by the high-voltage rail 9 previously described.This space resides w1thin the Electrons moving from dynode 6 toward theanode 7 follow a cycloidal path in response to the combined effect ofthe crossed magnetic and electric fields. Once an electron enters theelectric field-free space, it is subject only to the magnetic fieldproduced by magnets N and S and, therefore, the path of the electronbecomes circular. If the height d of this electric-field-free spaceextending normal to anode surface is greater than the radius of thecircular path the electron follows in this space, the electron will notreach the anode but will, following the circular path, pass a distanceabove the anode and head out away from the anode. It is, therefore, animportant consideration that the distance d, defining the outer limit ofthis space be less than the radius of the circular path that theelectron follows in the electric field-free space.

The radius of the circular path of the electron is a function of thevoltage of the electron and the strength of the magnetic field both ofwhich may be calculated for the particular tube application. By makingthe distance d less than the radius of the circular path of theelectron, the electron is assured of striking the anode surface.

Upon the impingement of the electron on the anode surface, a secondaryelectron is emitted therefrom. The secondary electron has considerablyless energy than the primary electron which liberated it, usually lessthan 10% of the energy of the primary electron. By virtue of its energyat liberation, the secondary electron will move away from the anodesurface and follow a circular path under the control of the magneticfield within the said space. The radius of the secondary electronscircular path will be considerably smaller than that of the primaryelectron due to its much lower voltage.

If the distance d is less than the radius of the circular path of thesecondary electron, it will pass outside the electric field-free spacewhere it will be subject to both the magnetic field and the electricfield and its circular path will become a cycloidal path which can causeit to move out of the anode area. If, however, the distance d is madegreater than the radius of the circular path of the secondary electron,there is no chance that the secondary electron may pass outside the saidspace and since it will follow a circular path, it must return to theanode, thus insuring no loss of energy. 7

It can be seen from the foregoing that for maximum efiiciency, thedistance (I must be less than the radius of the circular path of theprimary electrons and greater than that of the circular path of thesecondary electrons.

FIGURE 4 shows a Faraday cage 8 disposed a distance a from the anode 7.The cage 8 consists of a rectangular grid energized to the samepotential as the anode as shown by conductor 10, FIGURE 1. The cage 8 isshown to be substantially the same size as the anode surface anddisposed parallel thereto. This is not the only configuration that maybe used. It only represents a preferred embodiment. The cage 8 may beconsiderably larger than the anode or it may be slightly smaller. Itneed not be parallel to the anode surface as long as maximum and minimumdistances measured normal to the anode surface are within theabove-mentioned limits.

By energizing the cage 8 and the anode 7 to the same potential the spacetherebetween is made substantially free of electric fields, therebyleaving only the magnetic field in this area. The electric field abovethe surface of the grid will not be completely shielded by the cage 8but it will penetrate intothe cage area to some slight degree. In myexperiments, I have found that the percentage of penetration isinversely proportional to the distance d, reaching a maximum of lessthan 5% for the smallest useahle d.

The efficiency of the magnetic Faraday cage is measured not in terms ofthe number of secondary electrons which fail to return to the anodebecause this is substantially nil, but, rather, in terms of thegeometrical transmission of the cage. This is simply the ratio of openspace in the cage grid to the total area of the cage grid. Fine meshgrids are commercially available with geometrical transmission in excessof This means that more than 85% of the primary electrons reaching thecage pass through it and strike the anode while less than 15% impingeupon the cage grid and are lost.

FIGURE 4 is similar to FIGURE 3 except that the Faraday cage grid isdisposed at the outer edge of the electric field-free space. A primaryelectron is shown in a cycloidal path before it passes through the cagegrid 8 where its path changes to a circular path before it strikes theanode 7 causing a secondary electron to be liberated which moves in acircular path completely within the said space and then strikes theanode. If the secondary electron liberates a tertiary electron when itstrikes the anode, the energy of the tertiary electron would be so smallthat its circular path would have an extremely small radius.

As an example, I have tested my invention in an amplifier tube with amagnetic field of 522 gauss and an electric field of 7.68 kv./cm. Theprimary electrons entered the cage grid with energies of 450 volts wherethey entered circular paths having a radius of 0.053 inch. Most of thesecondary electrons had energies less than 20 volts, but using an upperlimit of 50 volts, the radius of its circular path was 0.018 inch. Thus,distance d must be between 0.018 and 0.053 inch. The d actually used was0.037 inch, substantially, the mean distance.

Other and further modifications will occur to those skilled in the artand all such are considered to fall within the spirit and scope of theinvention, as defined in the appended claims.

I claim:

1. In an electron stream apparatus in which the electron stream fiow isunder the control of crossed electric and magnetic fields, a secondaryelectron emission controlling device comprising: a pair of substantiallyparallel planar electrodes including a collector toward which the streamof primary electrons is directed and a grid disposed a predetermineddistance from the said collector in the path of the said electronstream; and means for energizing said collector and said grid tosubstantially the same potential, said predetermined distance beingintermediate the radius of the circular path of the primary electronswhen subjected to only the magnetic field, and the radius of thecircular path of secondary electrons liberated by the impingement ofprimary electrons on the said electrode when said secondary electronsare subject to magnetic field only.

2. A device as claimed in claim 1 and in which the said collector andthe said grid are coextensive.

3. A device as claimed in claim 1, in which said grid has a greater openarea than closed area.

4. A device as claimed in claim 1 in which said grid electrode has arectangular grid.

References Cited in the file of this patent UNITED STATES PATENTS2,141,322 Thompson Dec. 27, 1938 2,150,632 Ploke et al Mar. 14, 19392,445,811 Varian July 27, 1948 2,460,141 McArthur Jan. 25, 19492,992,360 Reverden July 14, 1961

1. IN AN ELECTRON STREAM APPARATUS IN WHICH THE ELECTRON STREAM FLOW ISUNDER THE CONTROL OF CROSSED ELECTRIC AND MAGNETIC FIELDS, A SECONDARYELECTRON EMISSION CONTROLLING DEVICE COMPRISING: A PAIR OF SUBSTANTIALLYPARALLEL PLANAR ELECTRODES INCLUDING A COLLECTOR TOWARD WHICH THE STREAMOF PRIMARY ELECTRONS IS DIRECTED AND A GRID DISPOSED A PREDETERMINEDDISTANCE FROM THE SAID COLLECTOR IN THE PATH OF THE SAID ELECTRONSTREAM; AND MEANS FOR ENERGIZING SAID COLLECTOR AND SAID GRID TOSUBSTANTIALLY THE SAME POTENTIAL, SAID PREDETERMINED DISTANCE BEINGINTERMEDIATE THE RADIUS OF THE CIRCULAR PATH OF THE PRIMARY ELECTRONSWHEN SUBJECTED TO ONLY THE MAGNETIC FIELD, AND THE RADIUS OF THECIRCULAR PATH OF SECONDARY ELECTRONS LIBERATED BY THE IMPINGEMENT OFPRIMARY ELECTRONS ON THE SAID ELECTRODE WHEN SAID SECONDARY ELECTRONSARE SUBJECT TO MAGNETIC FIELD ONLY.